1. Field of the Invention
The present invention relates to a thermal recording device for recording an image or other information on a thermosensitive recording medium with a laser beam emitted from a laser beam generator.
2. Description of the Related Art
Thermal recording devices for applying a thermal energy to a thermosensitive recording medium to record an image or other information thereon are in widespread use. Some thermal recording devices employ a laser as a thermal energy source for high-speed recording operation (see, for example, Japanese laid-open patent publications Nos. 50-23617, 58-94494, 62-77983, and 62-78964).
There has been proposed a thermosensitive recording medium for recording a high-quality image thereon. The proposed thermosensitive recording medium comprises a support coated with a coloring agent, a color developer, and a light absorbing dye. The thermosensitive recording medium produces a color of a density which is commensurate with the level of a thermal energy applied thereto. There has also been proposed a thermal recording device which employs a laser beam to record an image on the proposed thermosensitive recording medium. For details, reference should be made to Japanese patent application No. 3-62684 and Japanese laid-open patent publication No. 5-24219.
The proposed thermosensitive recording medium has a thermosensitive layer that is produced by coating a support with a solution which is prepared by dissolving microcapsules containing a basic dye precursor, a color developer, and a light absorbing dye into an organic solvent that is hardly soluble or insoluble in water, and then dispersing an emulsified material into the mixture.
The basic-dye precursor produces a color by giving electrons or receiving protons as of an acid. The basic dye precursor is made of a compound which is usually substantially colorless and has a partial skeleton of lactone, lactam, sultone, spiropyran, ester, amide, or the like, which opens or cleaves upon contact with the color developer. Specific examples of the basic dye precursors include crystal violet lactone, benzoyl leucomethylene blue, malachite green lactone, rhodamine B lactam, 1,3,1-trimethyl-6'-ethyl-8'-butoxyindolinobenzospiropyran.
The color developer is made of a phenol compound, an organic acid or its metallic salt, an acid material such as hydroxybenzoic acid ester, or the like. The color developer should preferably have a melting point ranging from 50.degree. C. to 250.degree. C. In particular, phenol that is hardly soluble in water or an organic acid, whose melting point ranges from 60.degree. C. to 200.degree. C., is preferable for use as the color developer. Specific examples of the color developer are disclosed in Japanese laid-open patent publication No. 61-291183, for example.
The light absorbing dye should preferably absorb less light in a visible spectrum range and more light especially in an infrared spectrum range. Materials for the light absorbing dye which meets such a requirement include a cyanine dye, a phthalocyanine dye, pyrylium and thiopyrylium dyes, an azulenium dye, a squalilium dye, a metal complex dye containing such as Ni, Cr, or the like, naphthoquinone and antraquinone dyes, an indophenol dye, an indoaniline dye, a triphenylmethane dye, a triallylmethane dye, aminium and diimmonium dyes, and a nitroso compound. Of these materials are particularly preferable those which have a high light absorption ratio in a near-infrared spectrum range from 700 nm to 900 nm because of the fact that semiconductor lasers capable of emitting near-infrared radiation are in practical use.
The thermosensitive recording medium produces no color when exposed to thermal energy whose level is lower than a certain threshold, so that the thermosensitive recording medium can be kept in stable storage.
However, the thermosensitive recording medium is not maintained at a constant temperature as it depends on the environment in which it is used. If a laser beam is applied from a laser beam source to the thermosensitive recording medium without any concern over the temperature of the thermosensitive recording medium, then color density irregularities are developed on the thermosensitive recording medium, making it difficult to record a desired image thereon.
While an image or other information is being recorded on the thermosensitive recording medium, the humidity in the atmosphere in various sections, such as a recording unit, a thermosensitive recording medium storage unit, etc., of the thermal recording device tends to vary. When the humidity increases, the water content of the thermosensitive recording medium increases, resulting in an increase in the sensitivity thereof. As shown in FIG. 7 of the accompanying drawings, provided a laser beam of a constant power level is applied to record an image on the thermosensitive recording medium, the higher the humidity, the higher the density of the color which is produced on the thermosensitive recording medium. Consequently, when the humidity varies, the sensitivity of the thermosensitive recording medium varies, making it difficult for the thermosensitive recording medium to always record a desired gradation image.